1. Field of the Invention
The invention relates to an X-ray imaging system, including
an X-ray source for emitting an X-ray beam for irradiating an object to be arranged within the X-ray beam so as to form an X-ray image,
an X-ray image intensifier tube, having an entrance screen for the detection of the X-ray image and the emission of electrons, an exit screen, and an electron-optical system for imaging the X-ray image detected on the entrance screen as an optical image onto the exit screen, and
adjusting means for adjusting the electron-optical system so as to image surface portions (r.sub.1, r.sub.2) of different size of the entrance screen onto the exit screen.
The invention also relates to an X-ray image intensifier tube and to adjusting means suitable for use in such an X-ray imaging system.
An X-ray imaging system of the kind set forth is known from: B. van der Eijk and W. Kuhl: "An X-ray Image intensifier with large input format"; Philips Technical Review, Vol. 41, 1983/84, No. 5, pp. 137-148.
The cited article describes an X-ray imaging system comprising an X-ray image intensifier tube which converts X-rays formed in a CsI entrance screen into blue light which releases electrons in a photocathode. Using four electrodes, said electrons are accelerated and imaged on an exit screen. The exit screen, having a comparatively small diameter amounting to a few cm, comprises a phosphor layer which is vapour-deposited on a fibre-optical system and in which the electrons cause luminescence. The optical image formed on the exit screen is picked up, for example by means of a television camera so as to be displayed on a television monitor or is recorded on a 100 mm photographic film. Depending on the dimensions of the object to be imaged, different diameters of the entrance screen of the X-ray image intensifier tube are used. For example, the imaging of the kidneys or the stomach requires a large surface portion of the X-ray image intensifier tube entrance screen which has a diameter of, for example 38 cm, whereas for the imaging of smaller objects, such as a single kidney, a smaller surface portion having a diameter of, for example 17 cm is required. When the image format is adapted, the entire exit screen is still utilized, so that the magnification is changed. When a smaller surface portion of the entrance screen of the X-ray image intensifier tube is used, preferably a diaphragm arranged between the patient and the source is adapted to the smaller image format, so that unnecessary irradiation of parts of the patient which are not to be imaged is avoided. This is important for limiting the detrimental effects of radiation on living tissue as well as for a reduction of the X-rays scattered within the patient which reduce the image contrast. For adaptation of an image format of 38 cm to 17 cm, it is necessary to vary the voltage across at least one of the electrodes between, for example 3 kV and 35 kV by means of the adjusting means, in this case being a variable high-voltage power supply. In order to prevent the defocusing which accompanies the switch-over from becoming visible, switching-over should take place within the accommodation time of the human eye which amounts to approximately 0.2 s. This means that the output resistance of the high-voltage power supply, being connected parallel to the capacitance of the electrode whereto the high-voltage power supply is connected, should be small. A small output resistance, however, has the drawback that the dissipation therein is high, which impedes miniaturization of the high-voltage power supply and is detrimental to the long-term stability of the high-voltage power supply.
It is an object of the invention to provide an X-ray imaging system in which annoying visual effects are avoided when the image format is switched over. It is also an object of the invention to provide an X-ray imaging system in which the service life of the X-ray image intensifier tube is not affected by the switching-over of the format, the dissipation in the high-voltage power supply is comparatively small, and the high-voltage power supply is stable.
To achieve this, an X-ray imaging system in accordance with the invention is characterized in that the adjusting means are arranged to image, when a setting of the image of a first surface portion is changed to a setting of an image of a second surface portion, at least a third surface portion, having a size between that of the first and the second surface portions, onto the exit screen.
When one or more surface portions of intermediate size are imaged on the exit screen in a well-focused manner during switching over of the image format, a gradual increase or decrease of the image format is obtained while the object image remains well-focused so that observation is also possible during the changing of the image format. This is important for medical applications where a radiologist wishes to see a permanent image of the patient and does not wish to be distracted by adjusting effects, for example image flicker.
The gradual adjustment of the image format is also attractive for dose control whereby the mean light yield on the exit screen of the X-ray image intensifier tube is kept constant and also for automatic gain control of the video signal of the television camera. During dose control the light yield on the exit screen of the X-ray image intensifier tube is measured and the voltage of the X-ray source is adapted, in dependence on the thickness of the patient so as to achieve a constant mean brightness of the exit screen. This is necessary for correct exposure when images of the exit screen are recorded on photographic film and also for adequate illumination of the video camera. Because the voltage of the X-ray source may not be excessively increased for comparatively thick patients, the mean brightness on the exit screen of the X-ray image intensifier tube will then be lower than for thinner patients. The mean brightness of the video signal formed by the televison camera is kept constant in such cases by an automatic gain control (AGC). In the event of a sudden change-over of the image format of the X-ray image intensifier tube, the brightness of the exit screen of the X-ray image intensifier tube varies and the dose control and the automatic gain control may give rise to image-disturbing adjusting effects. This is counteracted by well-focused imaging of one or more intermediate image formats upon a change-over of the image format.
Furthermore, the charge displacements are comparatively small during the gradual adaptation of one or more voltages of the electron-optical system of the X-ray image intensifier tube and "flashing" of the X-ray image intensifier tube is prevented. "Flashing" occurs when, upon a fast voltage variation, electrons migrate, via insulator parts of the X-ray image intensifier tube, from an electrode carrying a high voltage to an electrode carrying a low voltage. Some ionization may then occur in the residual gases in the X-ray image intensifier tube; this is observed as an annoying, image-disturbing flash on the exit screen.
An embodiment of an X-ray imaging system in accordance with the invention in which the electron-optical system comprises an electrode, a voltage U(r) of which determines a size r of the surface portion imaged onto the exit screen, in which the adjusting means comprise a power supply circuit and an input circuit connected to the electrode, and in which an output voltage of the power supply circuit increases or decreases in response to an input signal to be formed by the input circuit, is characterized in that in order to change the size r of the surface portion imaged on the exit screen, the input signal is adapted by the input circuit so as to exhibit a variation r(t) which is continuous to the eye during a time interval dT, adaptation being such that the output voltage is substantially equal to the voltage U(r(t)) at any instant within the time interval dT.
Upon adjustment of the image format, the output resistance of the power supply circuit is connected parallel to the capacitance of the X-ray image intensifier tube, the power supply circuit and the cable between the X-ray image intensifier tube and the power supply circuit. When the power supply circuit and the X-ray image intensifier tube are integrated in a single housing, there will be no cables between the power supply circuit and the X-ray image intensifier tube. Because of the parallel-connected capacitance and the output resistance, in the event of a step-like variation of the input signal for the power supply circuit, to be formed by the input circuit, the output voltage of the power supply circuit increases or decreases exponentially with an RC-time which is determined by the output resistance and the capacitance. The invention is based inter alia on the insight that instead of aiming for a minimum RC-time of the power supply circuit for the benefit of a fast and invisible switching behaviour, choosing a slower, visible format adaptation allows for a longer RC-time which, as a result of the use of a high output resistance, leads to advantages such as lower dissipation in the power supply circuit and long-term stability.
Instead of adapting the image format according to the image format variation associated with the exponentially varying output voltage of the power supply circuit in response to a step-like variation of an input signal formed by the input circuit, it is often desirable to adopt a different variation in time, for example an image format which linearly increases or decreases in time. It is not possible to choose an arbitrary image format variation r(t), because the output voltage U(r) of the power supply circuit has a given inertia which is determined by the output resistance. The image format variation r(t) must be so slow that the variation per unit of time of the output voltage U(r(t)) to be supplied by the power supply circuit is smaller than the variation per unit of time of the exponential output signal of the supply voltage circuit due to a step-like variation of the input signal: ##EQU1## Therein, .tau. is the RC-time of the power supply circuit coupled to the electrode and U(O) is the output voltage of the power supply circuit at the beginning of the image format variation. In the case of a linear format variation, for which r(t)=kt, the time derivative of U(r(t)) can be written as: ##EQU2## The function of the voltage U in respect of the image format r is known from measurements and calculations. When from an instant t=0 and for a voltage U(0) of 35 kV the format is linearly increased from 17 cm to 38 cm (k=21) in a time interval of 1 s, and the maximum value of dU/dr occurs at t=0 and is given by 4000 Vcm.sup.-1, it follows from the above formules for t=0 that .tau.=0.4 s. For this RC-time, the output voltage of the power supply circuit can still follow the linear format variation. For a capacitance C of the electrode amounting to 150 pF, an output resistance R of the amplifier circuit amounting to a few G.OMEGA. can be used; this is very advantageous in view of power dissipation and stability of the power supply circuit.
An embodiment of an X-ray imaging system in accordance with the invention is characterized in that the input signal to be formed for the power supply circuit (63) by the input circuit (59, 61) is stepped.
For an RC-time amounting to 0.4 s, as from an instant t=0 a linear format variation from 17 cm to 38 cm can take place within one second in a number of N discrete steps having a duration .DELTA.t, where in accordance with the formule (1): ##EQU3## Therein, U(0)=35 kV and .DELTA.U(0) is the voltage step of the output voltage per time interval .DELTA.t at the nstant t=0. The voltage step per unit of time is greatest for the instant t=0, so that the time constant .tau. associated with this value in accordance with the formule (3) is small enough to allow the power supply circuit to follow the further voltage variation U(r(t)). For .DELTA.U(0)=-377 V, the following value is found for .DELTA.t: .DELTA.t=1/232. The adaptation of the image format can take place in 332 equidistant steps.
A further embodiment of an X-ray imaging system in accordance with the invention is characterized in that the electron-optical system comprises several electrodes, each of which is connected to a respective power supply circuit whose input signals can be simultaneously adapted by the input circuit.
The X-ray image intensifier tube comprises, for example five electrodes whose voltage is to be adapted together in order to vary the image format. The X-ray image intensifier tube is notably made of metal and one of the electrodes is constantly connected to ground potential. The voltages across the five electrodes are stated for five image formats in the below Table. For adjustment of image formats intermediate of the five image formats, the voltages per electrode should be adjusted to the values of a voltage curve plotted through the five points. The electrode which is denoted as "anode 1" in the Table accelerates the electrons emitted by the cathode to a high speed; the electrode which is denoted as "anode 2" in the Table mainly determines the image format, the electrodes which are denoted as "focus 1" and "focus 2" have a focusing effect.
TABLE ______________________________________ electrode voltages for five different image formats. Anode 1 Anode 2 Focus 1 Focus 2 Cathode Size U U U U U [cm] [kV] [kV] [V] [V] [V] ______________________________________ 38 35.000 2.950 0 1800 -198 31 35.000 7.390 0 1150 -247 25,4 35.000 12.275 0 1115 -266 20 35.000 22.860 0 1275 -355 17 35.000 35.000 0 2000 -375 ______________________________________
A further embodiment of an X-ray imaging system in accordance with the invention is characterized in that the input circuit comprises a memory containing a Table in which voltage values are stored, for at least one electrode, for a number of N sizes of the surface portion of the entrance screen to be imaged, each size being represented by an address, and means for applying the electrode voltage values to the respective power supply circuits of the electrodes in a time sequence, the adjusting means comprising a format-adjusting circuit for receiving a format value of a new size to which the sub-surface to be imaged is to be adjusted and for supplying the memory with addresses associated with the sizes of the surface portions situated intermediate of the present size and the new size to be adjusted.
A user of the X-ray imaging system can apply a format value of an image format to be adjusted to the format adjusting circuit. A difference signal to be applied to the counter can be obtained by comparing the format value of the image format with a counter position which is a measure of the image format used. In dependence on the sign of the difference signal, the counter position is incremented or decremented until the difference signal is zero. The counter addresses the memory which stores the electrode voltage values which are associated with, for example 256 image formats. In response to each incrementation or decrementation of the counter, the counting speed being determined by the frequency of a clock connected to the counter, for each electrode a new voltage value is transferred from the memory, via a digital-to-analog converter, to the power supply circuit. The power supply circuit comprises, for example for each electrode with the exception of the electrodes carrying a fixed potential, an operational amplifier whose output is fed back, via a resistor, to the inverting input and whose gain is, for example 4000 times. After adjustment of the desired image format, if necessary, an analog fine adjustment signal can be applied to one or more electrodes in order to readjust the focusing to the desired accuracy.
A further embodiment of an X-ray imaging system in accordance with the invention is characterized in that the adjusting means comprise calibration means for determining a voltage variation as a function of the size of the surface portion for the at least one electrode, the calibration means comprising:
a variable power supply for applying a variable calibration signal to the power supply circuit of the at least one electrode in order to focus the image on the exit screen for different image formats, and
arithmetic means for receiving calibration signals associated with a focused image of the various image formats on the exit screen and for determining the voltage values associated with the number of N sizes of the surface portion to be imaged from the voltage values adjusted by calibration.
The calibration means enable a user of the X-ray imaging system to adjust a number of voltages to be used for well-focused imaging. The voltages associated with intermediate image formats are calculated by calculation of a voltage curve as a function of the image format by means of the arithmetic means. For each individual X-ray image intensifier tube, an optimum voltage variation for the specific user as a function of the image format is thus obtained.